The present disclosure relates to a gas fuel injector for a combustor in an engine. More specifically, the present disclosure relates to the joints of a venturi tube in the gas fuel injector.
A present thrust of gas-turbine engine technology seeks to attain reduced emissions of nitrogen (NOx) and hydrocarbon compounds. Prior-art techniques for accomplishing such reduced emissions almost invariably result in reduced thermodynamic efficiency or substantially increased capital costs.
NOx compounds are produced by reaction of the nitrogen in the air at elevated temperatures conventionally reached in the combustors of a gas turbine engine. NOx formation can be reduced by reducing the maximum flame temperature in the combustor. Injection of steam into the combustor reduces the maximum flame temperature in the combustor at the cost of thermodynamic efficiency. Penalties must also be paid in water use, including water treatment capital outlay and operating costs. The amount of steam injection, and its attendant costs, rises with the amount of NOx reduction desired. Some states and foreign countries have announced targets for NOx reduction that infer such large quantities of steam that this solution appears less desirable for future systems.
NOx compounds can be removed from the exhaust downstream of a gas turbine engine by mixing a reagent such as, for example, ammonia, with the exhaust stream and passing the resulting mixture through a catalyst before venting to the atmosphere. The catalyst encourages the reaction of the NOx compounds with the reagent to produce harmless components. This technique, although successful in reducing NOx compounds to target levels, requires substantial additional capital outlay for the catalyst bed, a larger exhaust system to provide room for the large catalyst bed, and spray bars to deliver the reagent into the exhaust stream. The on-going cost of large quantities of the reagent must also be borne.
The maximum flame temperature can be reduced without steam injection using catalytically supported combustion techniques. For example, a fuel-air mixture is passed through a porous catalyst within the combustor. The catalyst permits complete combustion to take place at temperatures low enough to avoid NOx formation. Several U.S. patents, such as, for example, U.S. Pat. Nos. 4,534,165 and 4,047,877, illustrate combustors having catalytically supported combustion.
Reduction or elimination of hydrocarbon emissions is attainable by ensuring complete combustion of the fuel in the combustor. Complete combustion requires a lean fuel-air mixture. As the fuel-air mixture is made leaner, a point is reached at which combustion can no longer be supported. The presence of a catalyst also permits combustion of leaner fuel-air mixtures than is possible without the catalyst. In this way, catalytically supported combustion aids in reducing both types of environmental pollution (e.g., NOx and hydrocarbons).
Placing a series of venturi tubes upstream of the catalyst may ensure fuel and air are thoroughly mixed in an efficient way. One problem, not completely solved by the referenced prior-art patents, is that thermal stresses are applied to the structural joints that attach each of the venturi tubes within the injector. Thermal stress to structural joints may cause cracks in the joints before the normal service interval. This reduces the service life of the injector causing increased maintenance costs and a decreased operational reliability.
Venturi tubes are typically brazed to the header plates, and the perimeters of the header plates are sealed to form a plenum into which pressurized gaseous fuel is supplied. The braze joints of the venturi tubes are normally exposed to cold flow from the fuel on one side and hot air flow on the other inducing a cyclic stress. Thus, it is desirable to thermally isolate the braze joints and thereby increase the service life of the injector.